In the field of drilling in the exploration for oil and gas, an important component is that of the formulation of drilling muds. Drilling muds are the fluids which are used to maintain pressure, cool drill bits and lift cuttings from the holes, and vary in composition over a wide spectrum. Generally, drilling muds are based on aqueous formulations or oil-based formulations.
A conventional water-based drilling mud formulation is comprised of basically the following ingredients: water, a clay such as bentonite, lignosulfonate, a weighting agent such as BaSO.sub.4 (Barite), and a caustic material such as sodium hydroxide and a caustic material such as caustic barite, to adjust the pH of the drilling mud to a pH of about 10 to about 10.5.
In addition to cooling the drill bit and sweeping out the drilling fines from the vicinity of bit, the muds are capable of imparting a substantial positive pressure on a formation through its high density. This latter feature is due to the addition of high concentrations of insoluble, solid, high density particulates (i.e., weighting agents) such as barite. However, these particulates inhibit the drilling rate and possibly damage a variety of underground formations. This problem becomes even more acute as the drilling fines are "introduced" into the mud. Therefore, there has been a substantial need for a homogeneous, high density drilling mud which exhibits good performance at both high temperature and high ionic strength.
As alluded to previously, a very desirable change in the formulation of a drilling fluid would be the elimination of all added particulates. One practical approach to this problem is to formulate a drilling fluid that is clear, homogeneous, dense, single phase and possesses the appropriate viscosity requirements (in general, 40 to 50 cps). Therefore, a water-based mud containing principally a polymeric viscosifier in a high concentration brine (weighting agent) could meet the above-stated requirements. Such a fluid would be quite economical since some processing steps (and materials) are eliminated. For instance, brine can be obtained directly at the drill site.
However, it should be pointed out that the ability of macromolecules to effectively viscosify a high ionic strength solution is generally poor, since the dimensions of the polymer chains tend to collapse under these conditions. This is especially true for polyelectrolytes (i.e., homogeneous-charged polymers). A collapse in the dimensions of the chain results in significant loss in viscosity. Therefore, it is imperative for successful use of polymers in high ionic strength solutions that chain expansion rather than contraction should take place. Polymeric materials composed of N-vinyl-2-pyrrolidone (NV2P), sodium styrene sulfonate (SSS) and methacrylamidopropyltrimethylammonium chloride (MAPTAC) were observed to enhance the viscosity of aqueous solutions containing high levels of salt, acid or, base. these materials meet the requirements for producing a homogeneous, single phase, high density, water-based drilling mud. The N-vinyl-2-pyrrolidone units impart a substantially improved high temperature stability to the drilling fluid due to their own intrinsic hydrolytic stability.
There has been substantial need for a water-based drilling fluid which would exhibit good performance at high temperature. Previous experience has shown that most polymeric viscosifiers are effective in salt-free (i.e., fresh water) systems; however, they lose their effectiveness upon the addition of salt. As the temperature is increased, the viscosity loss becomes even more pronounced. There is need, therefore, for a polymeric viscosifier which can maintain viscosity and gel strength in high ionic strength, weighing agent-free (or at low concentrations), water-based muds up to high temperatures (exceeding 300.degree. F.). These needs are not adequately met by the current viscosifiers.
This invention describes an approach to viscosification of water-based drilling muds which permits the substitution of N-vinyl-2-pyrrolidone-based polyampholyte terpolymers for amine clays and barite (weighting agent). The resulting polymer-modified drilling fluid displays rheological properties which are in a desirable range for drilling mud applications, based on tests conducted for 16 hours at a variety of temperatures.
The types of N-vinyl-2-pyrrolidone-based polyampholytes that are envisioned in the present invention include N-vinyl-2-pyrrolidone as the nonionic monomer unit and the following anionic and cationic species:
Anionic: 2-acrylamido-2-methylpropane sulfonic acid, sodium styrene sulfonate, (meth)acrylic acid, 2-sulfoethylmethacrylate and the like. PA0 Cationic: Methacrylamidopropyltrimethylammonium chloride, dimethyldiallylammonium chloride, diethyldiallylammonium chloride, 2-methacryloxy-2ethyltrimethylammonium chloride, trimethylmethacryloxyethylammonium methosulfate, 2-acrylamido-2-methylpropyltrimethylammonium chloride, vinylbenzyltrimethylammonium chloride and the like.
These monomers possess the appropriate water solubility so that polymerization can take place.
The preferred species of the instant invention is low to moderate charge density N-vinyl-2-pyrrolidonebased polyampholytes with approximately 1 to about 60 mole % ionic groups. A 1:1 molar ratio of anionic and cationic is not required for effective utilization of this polymer. It is found that these terpolymers are soluble (low charge density) or readily dispersible (moderate charge density) in fresh water systems. Homogeneous, clear solutions form with the addition of soluble acid, base, or salt showing that the polymer is readily soluble in these solutions. In addition, the viscosity increases with the addition of these solutes. As a consequence, these polymers are extremely effective viscosifiers in a high ionic strength, water-based mud, even at relatively low levels. Moreover, the hydrolytic stability of the N-vinyl-2-pyrrolidone moieties imparts a substantially improved high temperature stability to the water-based drilling fluid.